bug#62435: [PATCH] 29.0 Fix eww--download-directory on MS-Windows

[Eli Zaretskii]

> From: "lux" <lx@shellcodes.org>
> Date: Sat, 25 Mar 2023 19:56:43 +0800
> 
> The eww--download-directory hardcodes the download path, so it does not
> work on MS-Windows, I fixed it, and tested on MS-Windows.

Thanks, but this isn't the right fix.  The code does want to honor the
user's "Downloads" subdirectory, even on Windows.  In addition,
USERPROFILE only works on latest Windows versions.

I think the right fix here is to make this directory a defcustom, and
leave its default value at "~/Downloads" on all systems.